Extended projects 2019 cohort

Transtibial prosthetic socket design: Understanding the requirements for a healthy residual limb

Student: Kevin Alarcon
Supervisors: Eric Kim, Ole Thomsen, Elena Seminati (University of Bath), HaNa Yu (University of Bath)

A transtibial prosthesis is for those with an amputation below the knee. The prosthesis aims to restore the function of a lower leg partially; however, they are too often abandoned by their users. The reason for is a poor socket design. The socket is the part which interfaces between the prosthetic and the amputated limb. It has to transfer all the moments and loads during everyday movement and be biocompatible to keep a healthy limb. Current socket designs are not in synergy with the amputated limb. The lack of effective design might be due to the difficulty in holistically understanding and solving the design challenges because this requires multidisciplinary knowledge across biology, biomechanics, mechanical and manufacturing engineering, clinical knowledge, and experience.

This work aims to use a holistic approach for the design of a prosthetic socket of a mature residual limb of a transtibial amputee and assess current designs from a holistic perspective. The literature was used to derive a set of design requirements for maintaining a healthy limb. These requirements were used to analyse current socket designs. It was found that designs often focused on the pressure and shear distribution but failed at maintaining good temperature and moisture balance, which in turn significantly weakens the skin and hampers its functions. Finally, this work provides the insight needed to implement smart composite solutions in the design and manufacturing of prosthesis and thus enables the wide variety of requirements to be satisfied to a high degree.

Back to top

Enhancing modelling workflow: additional tools for LAMMPS users

Student: Matthew Bone
Supervisors: Terence Macquart, Ian Hamerton, Brendan Howlin (University of Surrey)

The complexity of molecular dynamics (MD) simulations mean significant pre-processing is required before a simulation can begin. The challenge of generating input files is daunting to beginner and experienced modellers due to the size and intricacy of these files. Many MD solvers, both commercial and open source, have tools to create molecules and automate parts of the pre-processing stage, however, the more developed these tools become the more niche their typical use is. The software package LAMMPS lacks many of these tools, but has found popularity due to its robustness, scope, and potential for expansion. LAMMPS users rely on third party tools, such as Moltemplate or Topotools, to handle file pre-processing. However, these tools still require significant effort on the part of the user. The present work presents several additional tools, that alleviate pre-processing efforts and automate parts of this existing software. Two of these Python projects revolve around Moltemplate: a tool that converts molecules from the .xyz file type to an initial Moltemplate input file, and a package of scripts that replicate the DREIDING force field within Moltemplate. The third tool enables rapid comparison of .xyz file geometries - a method used in validating the DREIDING force field. Validation results contrasting total energy of optimised structures are presented and show a percentage difference of < 1% in most instances. This evidence suggests that a replica of the DREIDING force field for Moltemplate has been produced. Although difficult to quantify, this collection of tools enhances the user's workflow and facilitates additional methods of analysis which would otherwise be taxing and error prone to achieve.

A MATLAB toolbox for the semi-empirical modelling of hydrogen sorption isotherms

Student: Charlie Brewster
Supervisors: Valeska Ting, Sebastien Rochat, Lui Terry, Ashkan Salamat (UNLV)

Hydrogen is widely acknowledged to be a promising renewable fuel for replacing petroleum and offers the prospect of clean and abundant energy. Within hydrogen storage, a common assumption of hydrogen adsorption above the supercritical temperature is that the maximum density is equal to the fluid phase. However, densities can often be considerably higher than that of the bulk phase hydrogen and can even exhibit short-ranged order due to confinement effects. Several reports of densities greater than solid-phase molecular hydrogen in microporous materials have emerged, dispelling previous assumptions, and providing an exciting avenue of study.

Semi-empirical modelling of experimental adsorption data provides a convenient and straightforward method for estimating the hydrogen density, which would otherwise be difficult, expensive or time-consuming to measure directly. Within this work, a versatile MATLAB function was created, facilitating the consistent estimation of the adsorbed hydrogen density through fitting a semi-empirical equation to an experimental adsorption isotherm. A second function was further created, taking the results from fitting to calculate the isosteric heat of adsorption, which indicates the strength of the interactions between the hydrogen and porous material. Therefore, modulating heat of adsorption through changing influential properties, such as pore diameter, surface composition, or surface topology may lead to greater hydrogen storage densities.

Both functions were applied to adsorption isotherms at varying temperatures for carbon nanotubes, titanium carbide-derived carbons and two different activated carbons. Results obtained from analysis provide useful insight into the material properties influencing the densification of hydrogen. The results from this project may be used to guide material selection to design and fabricate composite materials for practical hydrogen storage.

Back to top

Advanced high-fidelity modelling of woven composites

Student: Ruggero Filippone
Supervisors: Bassam Elsaied, Stephen Hallett

A novel hybrid mesh technique has been implemented in the view of a multi-scale high fidelity virtual modelling of woven composites. The proposed 2D mesh technique aims to generate a faithful geometric model to account of the debonding failures initiation at the yarn/matrix interface. The algorithm is presented for a two-dimensional representative volume element of a fibre reinforced composite. A conformal mesh around the interfacial region between fibre matrix is modelled using cohesive elements, while a tetrahedral tessellation fills the remaining area belonging to the matrix. Finally, in the internal fibre/yarn regions a structured mesh has been implemented. A square grid centred on the fibre’s centroid is employed to reduce the average skewness of the mesh, providing a higher mesh quality.

Back to top

Modelling and analysis of truss reinforced composite panels

Student: Chris Grace
Supervisors: Ben Woods, Mark Schenk, Terence Macquart, Michael Wisnom

The Wrapped Tow Reinforced (WrapToR) Truss is produced through a novel filament winding technique, and has already proven to be mass efficient and capable of withstanding more than double the loading capacity of similar structures. In the modern space and aerospace industry, mass efficiency is of ever increasing importance due to the current climate crisis and reliance on fossil fuels for producing lift. To investigate the potential mass saving and stiffness benefits possible through use of WrapTor truss reinforcement of composite panels, pre- and post-processors were developed to allow for initial parameter sweeps to guide future experiments. The model has shown that for midspan compression on a curved panel with circumferential oriented truss, panel widths of less than 0.363m and ctc widths of greater than 0.03m may show specific performance gains of 50% or higher, and curved panels under axial compression with an axial oriented truss may increase the first buckling mode for panel widths less than 0.417m.

Back to top

A comparative study of effective elastic properties of cellular solids with compliant joints

Student: Athina Kontopoulou
Supervisors: Giuliano Allegri, Fabrizio Scarpa, Bing Zhang

Cellular cores provide a unique combination of properties to sandwich panels, including high specific stiffness and high specific strength, as well as low thermal conductivity, which makes them ideal for load-bearing, thermal insulation and energy-absorbing functions. The mechanical performance of the core strongly depends on the cell wall way of deformation. Cores with cell walls exhibiting axial stretching can be ten times stiffer than those deforming primarily through bending or buckling with the same relative density. This work investigates the implementation of compliant joints in cellular solids and their effect on the effective elastic properties. The main objective is to investigate the feasibility of inducing stretch-dominated behaviour in cellular solids, thus tailoring the specific stiffness and Poisson’s ratio. A numerical homogenization method and Periodic Boundary Conditions (PBCs) based on the finite element analysis of unit cells are employed to predict the in-plane and out-of-plane properties. Conventional and re-entrant hexagonal honeycombs are considered as benchmarks to compare the effective elastic constants, with the addition of compliant joints. The elastic properties are compared to those predicted from the analytical approach by Gibson and Ashby (1997) to validate the numerical methods of modelling. Parametric studies are conducted to explore the effect of compliant joints in respect with different combinations of materials, angles, and wall thickness ratios. The results from these studies offer valuable insights into the design of honeycombs with tailorable stiffness and Poisson’s ratio.

Back to top

An investigation into the scale up of the HiPerDiF process and performance of aligned, discontinuous carbon fibre

Student: Chantal Lewis
Supervisors: Ian Hamerton, Marco Longana, Carwyn Ward

Sustainability is an essential focus for composite manufacturing, increasing the demand for re-manufacturing of waste fibres. Significant progress has been made on producing highly aligned fibre composite with mechanical properties comparable to that of continuous fibres. However, not much has been done to determine the quality control requirements required to aid manufacturability. In this study, a pre-analytical model is developed to demonstrate the relationship between fibre length, fibre alignment, and the laminate’s tensile modulus. The model confirms a relationship between the fibre parameters and the tensile modulus as well as validate previous HiPerDiF experimental findings. To add depth to the study, a commercial management study (CMS) is used to outline additional quality control expectations of manufacturing industries when using a similar material. It breaks it down in terms of most desirable and nice to have. It also shows the disparities between the supplier and the end-use customer in terms of quality expectations. In order to show control of the requirements outlined in the model and the CMS, suitable measurement techniques have also been identified. These techniques are capable of in- line inspection with the ability to provide data with high accuracy. This is the first study to consider the connections between the industry requirements, fibre parameters necessary, and process control techniques for an effective fibre alignment process. This current work aims to provide the building blocks needed for industrialisation.

The results attained during this study found the pre-analytical model was capable of predicting the tensile values within 3% of experimental values. It suggests that using longer fibres (6 - 10mm) provided better tensile properties for the composite as long as the fibres were highly aligned. A comparison of results from previous work also suggests a relationship between fibre length and alignment. The study has also outlined measurement techniques such as Optical Laser Line Scanning, which can be used in-line to improve the quality control of the re-alignment process.

Back to top

Dynamic tuning of thin-walled cylinders by Continuous Tow Shearing

Student: Calum McInnes
Supervisors: Rainer Groh, Alberto Pirrera, Eric Kim

In aerospace engineering thin-walled cylindrical shells represent an area of significant interest due to their wide-ranging applications, such as launch vehicle stages and fuel tank structures. Future heavy lift launch vehicles require high propellant mass fractions to deliver payloads of increasing size to orbit, derived by Tsiolkovsky’s rocket equation which indicates preference for low mass vehicle structures to allow for larger payloads to be delivered to orbit. This design requirement leads to an emphasis on high strength-mass and stiffness-mass ratio structures. Consequently, NASA has identified lightweight materials and structures amongst the highest priorities for next-generation space vehicles to enable future manned exploratory missions beyond Low Earth Orbit. Typically, structures account for 60% of a launch vehicle’s dry mass, and hence composite materials represent a logical avenue of research for structural light-weighting.

Tow-steered composites, laminates in which the fibres follow curvilinear reference paths are typically manufactured by Automated Fibre Placement (AFP). The AFP process is prone to process-induced defects due to the fundamental tow deformation mechanism. Instead, this project utilised the Continuous Tow Shearing (CTS) process for tow steering. CTS mitigates the process-induced defects of AFP by shearing instead of bending material tows. The shearing of material tows gives rise to an orientation-thickness coupling which can be exploited as integrated stiffening features on a CTS structure.

Tow-steered composites, laminates in which the fibres follow curvilinear reference paths, have shown proven benefits to the axial compression load case of launch vehicle structures. However, the loading of launch vehicle structures is not solely static, during flight a launcher is subject to significant dynamic loading arising from sources. Thus, this Research Project aimed to investigate the potential performance benefits to structures in the dynamic loading regime.

In this Research Project several stacking sequence designs were proposed by searches of available design space to increase structural resonant frequencies. The commercial Finite Element software Abaqus was utilised in conjunction with Python scripts to numerically model tow-steered structures and conduct computational searches. Additionally, MATLAB was employed to investigate the effects of spatial variation in directional stiffnesses on a structure. Resulting nonlinear stiffness variations directly influenced the dynamic response of the structures and acted to raise the resonant frequencies when compared to a straight tow design. All developed numerical tools confirmed the potential of utilising CTS composites to raise both the resonant frequency and averaged stiffness of thin-walled cylinders and hence give confidence to the necessity for future research.

Back to top

Topological optimization of composite beams with a graded lattice core

Student: Alex Moss
Supervisors: Alberto Pirrera, Terence Macquart, Ajit Panesar (Imperial College London)

Topology optimisation is a powerful tool in developing non-intuitive structural solutions. It is limited however by lack of wider integration with other light-weighting methods such as the use of cellular architectures. This combination could reduce weight further than either method can individually. In this project, the method for optimizing composite structures with graded lattice cores was planned and implemented on a simple cantilever beam problem. A density-based topology optimization algorithm was used to generate minimum compliance solutions to compare with a conventionally designed sandwich panel type structure. The solutions from the 2D problem showed upwards of a 3.6% improvement in compliance over the conventional design. 3D solutions were used to demonstrate the implementation of a repeated unit cell graded lattice to convert the density distribution to a manufacturable structure. The challenges associated with applying topology optimization to large aerospace structures were researched as this will direct further study of this topic.

Back to top

Design for 4D printing: Modelling of smart porous networks for in-vivo deployment

Student: Joe Surmon
Supervisors: Richard Trask, Kate Robson Brown

As a result of the recent breakthrough in additive manufacturing (AM), interest has been growing in the emerging topic of 4D printing; consisting of the additive manufacture of smart (adaptive) materials. 4D materials harness the rapid fabrication of additive manufacture with stimuliresponsive nature of smart materials. Unfortunately, there is little opportunity for the design and simulation of these materials in CAD (computer-aided design) software. 4D printing has yet to see the surge of accessibility and availability experienced by AM. Proof of concept modelling approaches are presented within 2D and 3D space. Firstly, the potential for pixel/voxel-based modelling is shown in the form of programmable shape changes by altered material distribution. Secondly, initial simulations are presented for the use of 4D materials in-vivo, particularly for the treatment of osteoarthritis. Finally, methods and arguments are proposed to open this field to non-engineers looking to design and manufacture: 4D printable, active, composite materials.

Back to top

Edit this page